Operation of sensing devices is based on several principles where measuring accuracy by each of them is more or less dependent on characteristics of signal transmission path and parameters of input circuits of the measuring devices such as relays, measuring units etc. Thus, load impedance mismatch and variation of additional stray or parasitic capacitances might influence the measurement accuracy of MV instrument transformers. It can also result in high sensitivity towards external electric and/or magnetic fields. In such cases, it is necessary to adapt signal outputs of each sensing device for each type of measuring device separately or include a special filter circuits at the input side of the measuring devices in order to ensure a sufficient signal integrity.
Present devices for addressing the already described issues include design of output impedances of sensing devices close to the input impedances of measuring devices or providing correction setting directly in a measuring device. Such solutions can become impractical with increasing numbers of measuring devices with different input circuit parameters. It is therefore known to use any of the following solutions:
1) Proper output impedance design—this solution is used in cases where more than just one type of measurement device is used. It is highly impractical because various output impedance designs are used for each of the measuring devices and special instructions are used to avoid sensing device damage due to the design confusion.
2) Correction setting of measuring device—this solution is used separately or together with the impedance design described above. The measured values can be adjusted (corrected) in a certain range due to the offset function in the measuring device. If various output impedances of sensing devices are considered, the correction settings must be performed for each of them. Where different lengths of connecting cables are used, an additional time for the configuration of the measuring device for certain variants is required.
3) Correction using additional passive components—in certain cases, additional corrections to improve sensing device accuracy are performed by using external passive components such as capacitors or resistors. However, the solution is still tailored to the specific applications.
The above-described methods to improve signal integrity of sensing devices and measuring accuracy show that where there is more variability of these devices, additional complexities in their configuration before or after measuring are required. Moreover, in the case of using only one signal from a sensing device to multiple devices at the same time, there are no known satisfactory solutions. All the above issues can be solved by using impedance matching elements as described herein.